1. Field of the Invention
The present invention is related logging while drilling oil well equipment, and, more particularly, to a method and apparatus for quantitatively determining variations of a formation characteristic after an event.
2. Description of the Related Art
The exploration for subsurface minerals requires techniques for determining the characteristics of geological formations. Many characteristics, such as the hydrocarbon volume, resistivity, porosity, lithology, and permeability of a formation, may be deduced from certain measurable quantities. Thus, the techniques for determining the measurable quantities must be accurate. There are several reasons for requiring accuracy in the measurements. For example, the measurements assist in evaluating the economics of a potential oil reservoir, and in determining the appropriate techniques for drilling the well.
Although the accuracy of measurements is important, there are many impediments to achieving satisfactory accuracy. At least one such impediment is caused by drilling and the uncertainties caused thereby. Ideally, all characteristics of an earth formation are known prior to drilling. One such characteristic is referred to as the true resistivity (RT) of the formation. The actual RT is not a measurable quantity due to Heisenberg""s uncertainty principle and the principle expounded by the Schrxc3x6dinger""s cat experiment, which both generally provide that an experiment does not have an outcome until the outcome is observed. Observing, then, alters any environment making completely accurate measurements impossible even for pristine environments. A drilling environment is far from pristine. For example, the drilling environment is exposed to drilling fluid, also known as mud, and the formation immediately alters due to contact with the mud. Changes caused by the mud include invasion changes due to the mud replacing fluid in the environment and absorption changes due to the environment absorbing the mud. The invasion changes alter any measurements, such as resistivity measurements of the affected environment. Changes to an environment may also be caused by other events, natural and man-made.
Furthermore, the changes caused by the mud are exacerbated partly because logging sensors are typically several feet behind the bit of a drilling string. Therefore, a length of time will pass between the bit cutting into a rock environment and the logging sensors measuring the rock environment. Prior art methods of determining original rock formations and environments fail to provide accurate information concerning the original, untouched environment. Because the reasons for drilling are to locate oil and gas reserves found in the virgin, undamaged environment, there is a need to determine as accurately as possible the original state of the environment and to identify changes caused by drilling that could be from the drilling and not related to the original state of the environment.
A method for obtaining quantitative characteristics of an area of investigation includes measuring characteristics of the area of investigation in a first dimension, coordinating the measured characteristics with an index of a second dimension, the coordinating enabling an identification of a trend of the measured characteristics, and extrapolating using the trend in the second dimension to obtain quantitative characteristics of the area of investigation.
In one embodiment, the first dimension is a depth dimension and the second dimension is a time dimension. Further, in one embodiment the first dimension is a depth dimension, the measuring being a measuring of a zone of interest, and the area of investigation is a well, the zone of interest being a depth zone. The method, in an embodiment, further includes choosing one or more measurement points within the area of investigation and plotting the one or more measurement points against the index of the second dimension to show changes of the characteristics of the area of investigation, the plotting providing quantifiable characteristics of the formation prior to the measuring.
One embodiment is directed to an apparatus configured for use in a drill hole environment. The apparatus includes a clock configured to receive data from the depth meter and a processor configured to correlate clock data and depth data to provide a time after bit measure associated with a plurality of measurements of the measurements taken by the tool whereby the measurements taken at different depths are useful as compared to measurements taken independent of the time after bit measurements.